I think I would ask "Why do you perform his music?"
That seems like a simple, almost too simple question, but quite honestly, it seems that not enough coductors perform him. Perhaps they haven't gotten around to it yet, or perhaps they avoid Sibelius like the plague. But it seems only a select few do Sibelius, and even a more select view can be considered "Sibelius specialists" (or a "Sibelian") like Vänksä or Davis.
So, in a world where other composers seem to get more play by various conductors, my question really means "what value do you see in his music to perform it?" "or "why or other conductors wrong when the avoid him?"